The invention relates to an adaptive absolute steering angle sensor for absolute determination of an angle of rotation, in particular for determining the steering angle in a motor vehicle by means of a code, provided over an angular range of 360xc2x0, for determining the angle. The code and a detector arrangement are arranged such that they can rotate relative to one another. Absolute determination of the angle is effected by the contrast information being read by means of a microprocessor which determines both the angular position of the code and the fine resolution of the angles as a result of the relative position of the identified code with respect to the image on the photodetector arrangement. At the same time, the overall function of the system is checked and adapted during each measurement.
The invention is based on the object of further improving the determination of the absolute angular position of a rotor, in particular of the steering wheel of a motor vehicle.
According to the invention, the code is determined by means of a photodetector arrangement provided at a single location and is used for determination of the angle. Furthermore, a contiguous segment of the code track is imaged onto at least one photodetector linear array, where at least one code word is detected and a predetermined angle corresponding to said code word and the position of the code word with regard to a fixed position of the photodetector linear array is measured.
For this purpose, the code is chosen in such a way that it is not repeated over the entire circumference in the observation region of the photodetector arrangement. The code is in a single track, unambiguous and closed. The sensor has the advantage that, compared with known methods, the angular resolution does not depend on he resolution of the code of the code track and does not depend on the number of code words, but rather only on the resolution of the sensors of the photodetector arrangement. That is to say that the angular resolution is independent of the code. Without the use of a reference mark, the angular resolution depends on the number of code words. If at least one code word of the code track is detected by the photodetector arrangement, an angular resolution of 1xc2x0 would be obtained given 360 code words.
Since the angular resolution does not depend on the number of code words, the fewest possible code words should be used, in order to reduce the sensor sensitivity to environmental influences, such as soiling. This is achieved, e.g., by the use of 6-bit or 7-bit codes instead of 8-bit codes.
The measurement is performed by software in a microcontroller which, for this purpose, uses the image data of the photodetector arrangement.
In order to increase the resolution further, at least two different images of one or more circumferential codes may be imaged onto the photodetector arrangement.
The code track may be transilluminated with parallel light or may be illuminated from one side.
In a further refinement, it is provided that the code is imaged onto the photodetector linear array via an optical arrangement in such a way that, with a reading cycle of the linear array, not only is the absolute angle information determined but also the overall function of the system is checked and adapted.
In order to monitor the system functions, at least one reference shadow image may be projected onto the photodetector linear array. In the event of soiling in the region of optical components, e.g., the power of the light sources can then easily be adapted by increasing the control current. The failure of individual detectors of the photodetector arrangement is also noted and can be compensated for by computational methods. The reference shadow images can be generated by corresponding software either cyclically or by a computer-controlled monitoring device being switched on individually.
Furthermore, it is possible that for the compensation of optical and mechanical tolerances, the edge steepness and image size of the signals imaged on the detector linear array are evaluated.
The angular range is determined within 0xc2x0 to 360xc2x0 by way of the traveling speed of the vehicle. In order to detect the absolute steering angle even with vehicle systems switched off, he steering angle is determined by briefly switching on the steering angle sensor in time intervals in which rotation of preferably greater than 180xc2x0 is not possible.
By virtue of the fact that a single photodetector arrangement can very rapidly evaluate the angle in the range of 360xc2x0, the system is suitable by simple co-registration of the 360xc2x0 exceedance and hence for a plurality of revolutions. In order to ensure that not only for the traveling mode, the system must be switched on momentarily in each case in the standby mode, the switched-on intervals being chosen in such a way that steering wheel rotation of greater than 360xc2x0 is not possible in this interval. Since the system, for the purpose of data transmission, has an interface with the vehicle computer, the vehicle speed can be accepted from there, in order to define the zero range of the steering angle, since, in normal vehicles, no steering angle over, e.g., +/xe2x88x9290xc2x0 from the zero position can be traveled above a certain speed.
In an adaptive absolute angle sensor, moreover, provision is made of at least one light source for illuminating an angular range of the code, and provision is made of a photodetector arrangement for detecting the illuminated angular range of the code, where a microcontroller is assigned to the light source and to the photodetector arrangement.
In a first embodiment, as a light source, two light-emitting diodes are arranged symmetrically with respect to the optical axis, these being provided together with the photodetector arrangement and an optical arrangement on the same side of the circular ring.
In a second embodiment, a circular coding ring which is optically transparent at light locations of the code is provided. Furthermore, at least one light-emitting diode is arranged on one side of he coding ring and the photodetector arrangement is arranged on the other side of the coding ring. An optical arrangement is preferably provided on the side of the light-emitting diode.
In order to increase the reliability of the angle sensor and in order to detect code errors, a further refinement provides for the photodetector arrangement to have two sensors, which are arranged vertically, one above the other with regard to the course of the code track for the synchronous observation thereof, and for the images of the two sensors to be compared with one another. The comparison of the two images makes it possible to identify local soiling particles (effecting the code or optical arrangement) and also sensor errors. Two linear array sensors or two portions of an area sensor lying one above the other can be provided as the sensors.
In a further refinement, two sensors are arranged horizontally next to one another for synchronous registering of adjacent code words of the code track.
Charge-coupled elements (CCD) are preferably provided as the photodetectors.
In order further to reduce the sensor sensitivity to environmental influences, the light-dark lines of the code words should have the largest possible dimensions. The light-dark lines of the code words preferably have a width of from 2 to 3 mm.
In ore embodiment, it is provided that a matte plate is arranged between the light source and the transparent coding ring, and that the optical arrangement and the photodetector arrangement are provided on the other side of the coding ring.
In a further embodiment, a transparent coding ring has, in portions, cylindrical lenses lying next to one another and serving to generate the code and image the light sources on the photodetector arrangement. An additional optical arrangement is obviated in this embodiment. Stripes of different brightness are produced on the photodetector array as a result of the cylindrical lenses.
In a further refinement, it is provided that the coding ring has a prismatic cross section, there being arranged a light source which radiates in the axial direction of the coding ring.
In a further embodiment, there are arranged on a transparent coding ring, in portions, cylinders or lenses lying next to one another and serving to generate the code and image the light sources on the photodetector arrangement, where the cylinders extend in the axial direction of the coding ring and are fixed by one end area on the coding ring and are assigned to the light sources and are assigned with their other, exposed end areas [sic] to the photodetector arrangement. By means of regions with and without cylinders and configuration of the density and respectively with and without lenses, a code can be applied and correspondingly detected. The exposed end areas of the cylinders are expediently plane or are of lenticular design.
In one embodiment, at least one point light source is assigned to the transparent coding ring, which has optically transparent regions and optically opaque regions. In a first refinement, two point light sources are provided next to one another at a constant distance and a single photodetector arrangement is provided. This arrangement has the advantage that despite the change in the radial distance of the coding ring due to radial runout of the steering wheel or despite a change in the distance between the components due to mechanical or thermal influences, on account of the different shadow formation of the two light sources whose distance remains constant, the position and angular position of the code on the photodetector arrangement can be detected accurately.
The same advantageous effect can be obtained as follows: one point light source and two photodetector linear arrays, which are arranged one above the other at different distances from the light source, are provided.
A further refinement provides for at least one reference code to be assigned to the angle-determining code on the coding ring. In this case, the reference code may be arranged next to the angle-determining code, or be provided above and below the angle-determining code.